1. Field of the Invention
This invention generally relates to the marking of metals with a plating formed from an electroplating solution. In particular, the invention relates to the local plating of metals onto an oxide scale.
2. Description of the Related Art
Electroplating is a known technique for the plating of conducting surfaces. In general terms, electroplating refers to the technique of depositing a metal layer onto a cathode through the use of a metal ion current. The ion current is established in response to a voltage generated between the cathode and an anode by an external power source. In some instances, the anode is at least partially comprised of solid metal atoms, which are oxidized by a potential difference and dissolve into an intermediate electrolytic solution. In other instances, metal ions are introduced directly into the electrolytic solution through, for example, the dissolution of metal salts into the solution. In either instance, the electric field between the cathode and anode causes the metal ions travel through the solution to the cathode, where the ions are electrically reduced and thus deposited onto the cathode surface as a solute of metal atoms.
Electroplating commonly is performed by placing the object to be electroplated, i.e., the cathode, in an electrolyte bath also containing the anode. For example, U.S. Pat. No. 5,246,786 discloses electroplating a SPCC-grade steel tube with a nickel plating. The electrolyte used by the '786 patent is a Watts-type bath. A Watts-type bath is a known electrolytic solution for plating nickel and is comprised of nickel sulfate, nickel chloride and boric acid in varying proportions, depending upon the physical properties desired of the nickel plate, e.g., conductivity and luster. In the '786 patent, prior to nickel plating, the steel tube is coated with 3 μm of copper.
One drawback of the bath electroplating method is that the entire surface of the object is plated. An electroplating method that overcomes this limitation and allows for the plating of localized areas of an object is brush plating. In the brush plating method, the anode partially comprised of an absorbent material, which contains the electrolytic solution and prevents a short circuit from occurring due to contact between the cathode and the anode. Electroplating is then performed by brushing the anode over the cathode. In this manner, a localized area of a larger surface may be electroplated. One example of brush electroplating is described by U.S. patent application Ser. No. 10/278,889, which discloses brush plating steel tubes with a nickel electrolyte for the purposes of in situ crack repair. In the '889 application, plating thicknesses of approximately 25 mm are achievable using a Watts-type bath, and the nickel plating is comprised of nanocrystalline nickel grains having a average grain size of 13 nm. Steels suitable for use in the process described by the '889 application include 4130 high-carbon, 304 stainless and 1018 low-carbon steels. U.S. patent application Ser. No. 10/516,300 discloses a process similar to that of the '889 application. In the '300 application, a graphite anode is used to brush plate nickel onto various metals; a Watts-type electrolyte is used, with nickel carbonate added at periodic intervals to maintain a desired concentration of nickel ions.
When performing an electroplating procedure such as those described above, however, certain limitations must be considered because electroplating cannot be carried out on an oxide layer. In an electroplating process, an electrically-conductive cathode is typically required; otherwise, the cathode can act as a capacitive element in the electrical circuit, preventing the flow of the metal ion current and effectively halting the electrochemical process. Thus, capacitive surface layers—in particular, oxide layers, as well as greases, oils, and dirt—generally must be removed from the cathode prior to plating. In many instances, these surface layers should also be removed to facilitate adhesion of the plating to the cathode. For example, the '889 application describes the use of alkaline cleaners to remove dirt, oil, and grease from the cathode, followed by the use of an activation solution to remove any surface oxides. The electroplating apparatus used to perform the process disclosed by the '889 application includes pathways for the flow of these surface cleaning and activation fluids.
As another example, the '786 patent uses an intermediary layer of copper coating onto which nickel is plated. Therefore, in the '786 patent there is no need to activate the surface in the manner described by the '889 application. However, although the '786 patent may describe electroplating onto steel without removal of the native oxide, the workaround proposed is unwieldy; deposition or formation of a copper coating prior to electroplating can increase the cost, time, and labor required to electroplate the steel. Depending upon the size of, placement of, or environmental conditions around the steel part, deposition of a conductive layer prior to electroplating may even be impossible.